Super capacitor, or ultracapacitor, or supercondenser is an energy storage device of high efficiency, which can be either electric double-layer capacitor, redox capacitor or hybrid capacitor. Present invention is related to electric double-layer capacitor in which energy is stored in the surface of carbon electrodes with electrostatically large surface area, in the so-called electric double layer. Capacitor of that kind is characterised by a very rapid charge-discharge cycle, from few minutes to few seconds. For simplicity's sake the electric double-layer capacitor shall be henceforth called super capacitor. In order to achieve the good energy and power output parameters of the super capacitor the composition of its components needs to be well optimized. A significant factor is the compliance of electrode materials with each other, i.e. microporous carbon of large surface area and the electrolyte. There is a general rule, according to which the smaller the pores, or gaps are in the carbon of the electrode, the higher is the apparent density of corresponding carbon and the higher can be the maximum double-layer capacity of given super capacitor. The more accurately the electrolyte ions fit into the so-called pores, the bigger specific capacity and energy density is achieved with given carbon electrodes. On the other hand, it is known that if the ratio of the size of ions and pores approaches one, the diffusional resistance increases significantly and, from certain moment onwards, adsorption energy exceeds the energy required for ion desorptsion within the voltage range employed at discharging. So-called screening out of ions from the electrolyte solution shall take place. As a result of that, internal resistance of the electrochemical system increases and power output properties of the capacitor deteriorate. Methods of preparing micro/mesoporous carbon materials with optimised pore size distribution are described for example in [WO 2004/094307] and [WO 2005/118471].
The optimisation of porosity and pore sizes of microporous carbon according to the dimensions of the electrolyte ions is known from prior art [WO 02/39468]. Carbidic carbon of varying nanostructure and pore distribution allows balancing the positive and negative electrode in electrode pairs and will also provide novel opportunities for optimising the electrolytes' composition and the co-existence of the electrolyte and electrode pairs according to the desired energy and power output properties of the super capacitor.
On the other hand, the smaller the carbon material's pores are, the smaller must be the electrolyte's ions. Ion dimensions can be significantly reduced by polar environment, i.e. a solvent or mixture of solvents with high dielectric permittivity. The “dilution” of high-polarity solvents with fluids of low viscosity in order to achieve through reducing viscosity a better mobility of electrolyte ions in a polar condensed environment is known from prior art (e.g. [U.S. Pat. No. 5,888,673], [U.S. Pat. No. 6,783,896], [U.S. Pat. No. 6,787,267]).
In order to achieve the maximum energy density and specific capacity of a super capacitor it is important to balance the positive and negative electrode capacities in the electrode pair. Balancing of electrodes by masses is described for example in US2006/0148112, MAXWELL TECHNOLOGIES INC., Jun. 7, 2006. The super capacitor described in current invention achieves superior energy and power output properties by balancing the thicknesses of carbidic carbon composite electrodes of varying specific capacity. The pores on a negative electrode are bigger and the electrode has inasmuch smaller density as the positive electrode, which is necessary for achieving the good mobility of ions and low internal resistance of the electrochemical system. While the specific capacity of the negative electrode is somewhat smaller than that of the positive electrode, it leads to employing negative electrodes, which are up to 10% thicker than positive electrodes in order to equalise the electrode capacities.
An important requirement for achieving the low internal resistance of the super capacitor is the low charge transport resistance between the carbon electrode and current collector. Abrading of current collectors and improving the bonding by a carbon layer are known in prior art as the mechanical treatments of the aluminium foil layer, used as the mechanical current collector, described for example by PCT/US2009/050324 and PCT/US2009/050122. For improving the electric conductivity between the carbon layer and the current collector an electrically conductive and adhesion-improving intermediate layer is used, which is normally a polymer (e.g. polyvinylidene fluoride) including carbon nano particles (e.g. lampblack, nanographite, etc.). For reducing the resistance of charge transport between carbon and current collector in carbon fabric electrodes, deposition of aluminium on the carbon fabric is used, as described by US2005/0057888. Current invention describes covering one surface of a powdery pressed carbon composite electrode with a layer of aluminium by a special covering method, comprising a plasma-activated physical vacuum deposition method, which increases the mechanical strength of carbon composite electrodes, while also providing the maximum electrical contact surface between the topmost particles of the carbon layer and surface of the current collector. Aluminium particles deposited on the carbon film penetrate at pressure contact the ultra-thin non-conducting oxide layer on the aluminium foil, which acts as a current collector, so the prior abrasion or other mechanical/chemical treatment of the foil surface is not required.
Solid electrochemical package, wound from continuous electrode, as it normally exists in cylindrical super capacitors, entails a risk that the negatively and positively charged electrodes facing each other in the pair of electrodes have shifted in relation to each other during packing. This can cause at electric cyclisation of the electrochemical element the constant “overloading” of the electrode not aligned with the counterpart of the opposite charge, which can result in greater self-discharge, electrolyte disintegration and partial deactivation or disintegration of the electrode and rapid deterioration of super capacitor properties. As a solution to this problem, prior art (US2009/0180238) provides balancing of shifted electrodes or electrodes that do not coincide due to technological defects by removing a layer of non-coinciding activated carbon by mechanical treatment. Prismatic super capacitor, being the object of present invention, comprises a package of several flipped electrode pairs, connected electrically in parallel, in which the shifting of electrode pairs at flapping occurs with lower probability and, if it does, the few substandard packages can be replaced by ones of good quality, without having to dispose of the entire electrochemical system of the super capacitor. Also, it is important to consider in assembling the electrode pairs and flipping into packages that the carbon electrode layer that has no counterpart would not become the outer layers of packages, as it would increase the super capacitor self-discharge. Thus, when compared with cylindrical capacitors, the cylindrical capacitor according to the invention holds fewer chances of defects and occasional substandard electrode pairs occurring in the super capacitor, while the product quality, reliability and lifetime are improved.